Throughout the world numerous people and animals suffer from a variety of eye diseases. Three prevalent eye diseases of both humans and animals are dry eye syndrome, eye cataracts and nuclear sclerosis of the eye lens. The initial effects of each of these eye diseases may be mild, however, if left untreated these diseases result in substantial morbidity and even complete vision loss.
One common eye disease among both humans and animals is dry eye syndrome. Dry eye syndrome is a general term used to describe a heterogeneous group of diseases resulting from inadequate wetting of the cornea and conjunctiva by the precorneal tear film (PCTF). Dry eye syndrome conditions are classified as various types of abnormalities that can lead to insufficient wetting of the corneal surface. The five major classifications are: (1) abnormalities of the aqueous layer, (2) abnormalities of the mucin layer, (3) abnormalities of the lipid layer, (4) abnormalities of the corneal epithelium and (5) abnormalities of the eye lids. Abnormalities in the aqueous layer caused by decreased tear production result in a condition known as keratoconjunctivitis sicca (KCS). Deficient mucin production results in uneven tear distribution whereas abnormalities of corneal epithelial morphology adversely affect tear film stability. Deficiencies in the lipid layer result in excessive tear evaporation and similarly, eye lid abnormalities result in excessive drying of the ocular surface.
The most common treatment for dry eye syndrome is the use of an artificial tear solution. Enhanced artificial tears containing zinc and bicarbonate have been developed to aid in the regeneration of mucin, tear film and epithelium. The main disadvantage to artificial tears is that they must be used several times per day.
Eye cataracts (cataracts) are defined as any opacity in the lens of the eye. The normal lens is translucent (clear) and it transmits and focuses light onto the retina in the back of the eye. A cataract within the lens may block the transmission of light to the retina. In both humans and animals, cataracts cause varying levels of vision impairment and may lead to blindness. Another eye disease that results in an opacity of the lens but which does not cause blindness is nuclear sclerosis of the eye lens. The incidence of nuclear sclerosis increases with age.
In the United States, a significant proportion of both people and animals are diagnosed with cataracts each year. For example, about 9.79 million dogs and 1.17 million cats have cataracts. In humans, cataracts are a leading cause of blindness. About 1 in 7 cases of blindness in adults over 45 can be attributed to cataracts (Statistics on Vision Impairment: A Resource Manual, 5th edition, April 2002). Cataracts are also a leading cause of blindness worldwide. According to a World Health Organization Survey conducted in 1997, cataracts account for an estimated 16 million cases of blindness worldwide (Blindness and Visual Disability: Major Causes Worldwide, World Health Organization, 2001).
Currently, there is no medical treatment that is able to reverse cataracts. The most common treatment for cataracts is surgical removal. There are three principle surgical techniques that are used for cataract removal. The most common procedure is known as phacoemulsification. This technique has been widely performed on both humans and animals. In cases where a cataract is so hard that is cannot be efficiently phacoemulsified, a second technique known as extracapsular lens extraction is often used. In cases where a cataract is no longer held firmly in place in the eye, a technique known as intracapsular lens extraction is preferred.
Although cataract removal surgery has a high success rate, there are also several disadvantages. For example, when the eye is healthy, surgical procedures are usually effective, however, the success rate of surgery is greatly decreased if the eye is inflamed previous to or at the time of surgery. Moreover, complications associated with surgery, such as anterior uveitis and glaucoma are common. For example, in one study of 220 dogs (representing 346 eyes) that underwent cataract removal surgery, 134 of the 346 surgically treated eyes (38.8%) developed glaucoma within one year (Biros, et al. 2000. J. Am. Vet. Med. Assoc. 216:1780-6). In addition to such near-term complications, there are long-term complications of cataract removal surgery which are generally manifest as scarring of the lens capsule and detachment of the retina.
In addition to the above-mentioned surgical complications, cataract removal surgery is a relatively expensive eye surgery. For example, cataract removal surgery for one eye, including intraocular lens placement, typically costs about $2300.
In view of the above disadvantages associated with the current approaches for treating or reducing the symptoms associated with eye diseases, there exists a need for improvement.